This invention relates generally to magnetic field gradiometers and in particular to a fiber-optic magnetic field gradiometer for measuring first, second, and higher order derivatives.
In the detection, identification and location of tactical and strategic targets such as trucks, mines and submarines, one of the most difficult signatures to hide is the magnetic one. Even when visible methods are fogged over or radar methods are jammed, the magnetic signature indicating the existence and location of the targets still exists. Of particular interest is the location of targets whose exact position is unknown. Ferromagnetic hardware such as tanks and trucks on the ground, and submarines and mines under the water disturb the local geomagnetic field. These disturbances, or magnetic anomalies, are detectable as changes in the magnetic field measured as a sensor flies over them. For this reason, a sensor measuring the field changes or gradient provides more useful information than one merely measuring the field itself.
Present state-of-the-art for magnetic anomaly detection employs super-conducting quantum interference devices (SQUIDS) or proton precession devices. The latter devices do not have sufficient sensitivity to detect smaller targets such as trucks at sufficient range (1/2 kilometer) as a sensor flies over them, and require laborious calibration procedures. The former devices have a high resolution but, due to their superconducting nature, require large bulky devices for cooling.
K. P. Koo and G. H. Sigel, Jr. in "A Fiber-Optic Magnetic Gradiometer," (Journal of Light Wave Technology. Vol. Lt-1, No. 3, September 1983) disclose a fiber-optic gradiometer capable of measuring both AC and DC magnetic field gradients and is incorporated herein by reference. The concepts disclosed are shown graphically in FIGS. 1-3. FIG. 1 shows the flight trajectory of a magnetic field gradiometer as it flies at a height Z of 100 meters over three 1-meter radius iron spheres 1, 2 and 3 located on the X-axis at -75, 0 and 75 meters, respectively. FIG. 2 shows a graph of the magnetic field disturbances along the X-axis caused by spheres 1, 2 and 3 with the circles 1a, 2a and 3a indicating the respective spheres. FIG. 3 shows a graph of the first derivative or the magnetic field gradient of the magnetic field with spheres 1, 2 and 3 again indicated by circles 1a, 2a and 3a, respectively. However, as can be seen in FIGS. 1-3, no information on the location of the individual spheres in the array can be derived from the teachings of the prior art.
Accordingly, it is an object of the present invention to provide a method and apparatus capable of accurately determining location of a target.
It is a further object of the invention to provide an apparatus that is small and lightweight.
Other objects and advantages of the invention will become more apparent hereinafter in reference to the detailed description of the preferred embodiments and drawings.